In our initial studies of the regulation of RORgamma-t transcription we showed with luciferase reporter constructs driven by RORgamma-t promoter fragment of various lengths that Runx1 binding to the promoter is essential for optimal RORgamma-t transcription. In addition, we showed that E-proteins, transcription factors important during early T cell differentiation in the thymus, were also important transcription factors. In further studies in which we evaluated E-proteins in physiological cells by either transduction of retroviruses expressing these proteins or with gene silencing studies using E-protein-specific siRNA and found that these proteins also served as important RORgamma-t transcription factors. Of interest, high concentrations of E-protein suppressed rather than enhanced RORgamma-t transcription. However, this negative effect is probably ID2, a transcription factor that prevents E-protein binding to DNA. ID2 is induced by IL-6, a factor necessary for IL-17 expressison. The effect of E-proteins on IL-17 and RORgamma-t expression was verified in extensive studies of conditional E-protein KO mice. These mice had phloxed E-protein genes and a T cell-specific Cre transgene under a tamoxifen responsive promoter. Thus, when the mice or cells from the mice were exposed to tamoxifen the E-proteins were not expressed. In a final set of studies we found that E-protein levels were greatly enhanced by a combination of IL-6 and TGF-beta but neither of these alone. This finding offers a good explanation of why these cytokines are essential to IL-17 differentiation.